There are a variety of apparatuses for measuring the concentration of certain gaseous components of a gas mixture. One simple apparatuses, referred to as gas detector tubes or gas indication tubes, typically comprise a glass or other transparent tube and a chemical reagent that is capable of reacting with a target chemical compound resulting in a color change of the reagent.
Conventional gas detector tubes are glass tube filled with a chemical reagent that reacts to a specific chemical or family of chemicals. The chemical reagent is sealed within the glass tube and retained in position by gas permeable plugs on either end of the glass tube. In some cases, the chemical reagent may be liquid impregnated into a porous chemically neutral solid substrate. The chemical reagent is protected from exposure to contaminants and chemical compounds by sealing the tubes at each end until use and, therefore, extending the shelf life. To use the gas detector tube, the tips are broken to open a flow path through the tube and across the reagent. The air to be sampled may then be drawn through the tube and in contact with the reagent using a fixed volume sampling pump, for example. The reagent layer is capable of rapidly reacting with the target chemical compounds as the air to be sampled is drawn through the tube. The amount of reaction and the change of color of the reagent are related to the concentration of the target chemical compounds in the sampled gas and the volume of gas drawn through across the reagent.
To determine the gas concentration, a known volume of gas may be drawn into the tube and the concentration of the gas is the only variable. The length of the color change or the degree of color change of the reagent then corresponds to the concentration of the target compounds. Detector tubes that measure gas concentration by length of stain or length of color change are reliable and simple to use. Currently, detector tubes relying on measurement of the intensity or density of the generated color are not used because of difficulties in creating an appropriate set of color standards to indicate the concentration of the gas.
After manufacturing a batch of length of stain detector tubes, known concentrations of target gases are passed through the gas detector tubes to develop a batch specific a calibration curve relating the length of stain to a corresponding gas concentration. The calibration curve is included with the detector tube to allow visual reading of the concentration of a gas in a sampled volume. From their first introduction the detector tubes have their scale printed separately, see, for example, U.S. Pat. No. 2,174,349 to J. B. Littlefield. As the leading edge of color change of the reagent in the detector tubes is not always well defined, the scale divisions may be marked having a distance greater than length of diffusive front of discoloration. As such, the scales are of poor resolution and, more recently, the scales printed directly on the surface of the tube. For example, Dreager™, Gastec™, Kitagava™, Auer™, MSA™, RAY™, as well as other manufacturers of detector tubes have on their tubes the beginning of the scale—first scale division, marked 3 to 5 mm from the end the first input plug. Because of possible channeling effects, resulting in different lengths visible on each side of the tube, the divisions are printed as rings around the tube and numbers representing concentrations are in close proximity or into broken portion of the ring. The drawback of the known art is that such detector tubes cannot be read by electronic device because of the concentration lines, concentration amounts, and other marks on the tubes obstructing optical reading of the length of stain. For example, the markings may be interpreted by the device as a color change.
There has been a long felt need for a better more accurate and objective way of reading gas concentration with gas detector tubes. Heim et al. in U.S. Pat. No. 4,123,227 show a length-of-stain tube electronic reader based on detector tube without any printed matter. The detector tube serves as an alarming device and is periodically interrogated over a period of time. Leichnitz et al. in U.S. Pat. No. 5,069,879 suggested a tube having no scale on the readable part of the surface and printed means introducing into electronic reader all specific data for the tube including calibration data. There is a significant drawback of the tubes manufactured to be read by electronic reader only; they may not also by read visually.
The contemporary art of colorimetric reading devices is developed in the direction of devices even more specifically designed for optic-electronic reader. U.S. Pat. No. 5,089,232 to May shows an arrangement of tube-like devices for only electronic reading. U.S. Pat. No. 5,397,538 to Stark et al., U.S. Pat. No. 5,415,838 to Rieger et al. and U.S. Pat. No. 5,464,588 to Bather at al. depict development of specific tube-like devices for electronic reading of a zone of discoloration. Such devices however are highly specific and cannot be read without specialized electronic means.
All color change indicated by colorimetric reactions of the reagent depend to some extend on the ambient conditions—temperature, relative humidity and barometric pressure.
Temperature and relative humidity correction factors are typically provided by the manufacturer for a specific reagent and target compound. Like calibration curves the correction factors may be calculated on a batch basis. The correction factors should be applied to the concentration indicated by the length of stain in the detector tube and to more accurately determine the actual target gas concentration. The atmospheric pressure has two components altitude above sea level and weather factors. The altitude component is generally a larger factor in determination of the atmospheric pressure than the weather factors (usually much less than 1%). Weather factors may be considered negligible. The altitude of the gas detector above sea level, however, may have a significant outcome on determination of the actual concentration. In some cases, the measured concentration may be considerably below of the actual concentration.
TABLE 1Atmospheric Pressure Function of AltitudeAltitudePressure(km)(mb)Correction0.01013.25100.0%0.5954.6194.2%1.0898.7688.7%1.5845.5983.4%2.0795.0178.4%2.5746.9173.7%3.0701.2169.2%3.5657.8064.9%4.0616.6060.8%4.5577.5256.9%5.0540.4853.3%5.5505.3949.8%
Typical detector tubes also show an increasing sensitivity to color change with an increase in temperature. Therefore, gas detector tubes may also require compensation for temperature. The following table, Table 2, shows the temperature compensation factors for a Gastec™ gas detector tube for determination of the concentration of 1, 1 trichloroethylene in air:
TABLE 2Temperature, ° C.0.010203040Correction factor1.41.31.00.80.65
The following table, Table 3, shows the compensation factors for a particular gas detector tube manufactured by Gastec for determination of the concentration of hydrazine in air:
TABLE 3Relative Humidity %1030507090Correction factor0.80.91.01.21.4
Determination of a more accurate estimation of an actual concentration using gas detector tube should incorporate one or more of these compensation factors for environmental conditions. There is a need for a measurement system that provides automatic compensation of these parameters.